Heat-generating electronic or electric parts such as power transistors and thyristors deteriorate their performance due to the heat generated. It is a common practice in the prior art that such heat-generating electronic or electric parts are provided with heat sinks for heat dissipation or suitable means for conducting heat to a metal chassis of the associated equipment for heat release. To improve both electrical insulation and heat transfer, electrically insulating sheets of silicone rubber loaded with thermal conductive fillers often intervene between heat-generating parts and heat sinks.
As the thermal conductive, electrically insulating material, JP-A 47-32400 discloses an electrically insulating composition comprising 100 parts by weight of synthetic rubber, typically silicone rubber and 100 to 800 parts by weight of at least one metal oxide selected from beryllium oxide, aluminum oxide, hydrated aluminum oxide, magnesium oxide, and zinc oxide.
As the thermal conductive material for use in areas where electrical insulation is not required, U.S. Pat. No. 4,292,223, U.S. Pat. No. 4,292,224, U.S. Pat. No. 4,292,225 and U.S. Pat. No. 4,293,477 disclose a composition comprising 100 parts by weight of an addition curing type silicone rubber and 60 to 500 parts by weight of a thermal conductive powder such as silver, gold or silicon.
These thermal conductive materials, however, have a thermal conductivity of less than 1.5 W/mK. If silicone rubber compositions are loaded with large amounts of thermal conductive fillers in order to improve the heat transfer thereof, the compositions lose fluidity in the case of liquid compositions and increase plasticity in the case of millable compositions. In either case, the compositions become very difficult to mold and work.
One solution to this problem is found in JP-A 1-69661 which discloses a good thermal conductive rubber/plastic composition loaded with alumina consisting of 10 to 30% by weight of alumina particles having a mean particle size of up to 5 .mu.m and the balance of spherical corundum of single particles having a mean particle size of at least 10 .mu.m and of cutting edge-free shape. Also, U.S. Pat. No. 5,352,731 discloses a thermal conductive silicone rubber composition comprising 100 parts by weight of a base of an organopolysiloxane gum having an average degree of polymerization of 6,000 to 12,000 combined with an organopolysiloxane oil having an average degree of polymerization of 200 to 2,000 and 500 to 1,200 parts by weight of spherical aluminum oxide powder.
However, in the case of high loading of more than 1,000 parts by weight of aluminum oxide powder or more than 70% by volume of aluminum oxide, for example, even these methods relying on a combination of particles or a viscosity adjustment of the silicone base encounter a certain limit in improving the moldability and workability of silicone rubber compositions.
In electronic machines such as personal computers and CD-ROM drives, IC chips including LSI and CPU are increased in the degree of integration. Since such closely integrated IC chips generate more amounts of heat, conventional cooling means including heat sinks and cooling fans are sometimes unsatisfactory. In particular, portable notebook type personal computers are difficult to built in heat sinks or cooling fans because only a limited space is available inside. In such machines, IC chips are mounted on printed circuit boards which use as the substrate glass-reinforced epoxy resins and polyimide resins characterized by poor thermal conduction. It is then ineffective to release heat to the substrates through thermal conductive, electrically insulating sheets as in the prior art.
Then, heat-dissipating parts of air cooling or forced cooling type are disposed in proximity to IC chips so that the heat generated in the chips is conducted to the heat-dissipating parts. When the heat-dissipating part is in close contact with the IC chip, heat transfer is retarded due to surface irregularities. When a thermal conductive, electrically insulating sheet intervenes between the heat-dissipating part and the IC chip, the less flexibility of the insulating sheet allows the differential thermal expansion between the chip and the part to apply stresses to the chip, resulting in chip failure.
Additionally, the attachment of a heat-dissipating part to each circuit chip requires an extra space, preventing size reduction. A system capable of cooling a plurality of IC chips with a single heat-dissipating part is employed in such cases.
In particular, CPU's of the BGA type used in notebook type personal computers require deliberate consideration of a cooling system because they have a reduced height, but an increased heat release as compared with ordinary CPU's.
Where semiconductor chips of different heights are arranged with gaps therebetween, a low hardness, good thermal conductive material capable of filling the gaps becomes necessary. There is a demand for a thermal conductive sheet having a high thermal conductivity and flexibility and being compliant with gaps of differing size and shape. As the drive frequency becomes higher, CPU's improve their performance, but produce larger amounts of heat. A better thermal conductive material is desired in this regard too.
JP-A 2-196453 discloses a sheet molded from a silicone resin loaded with a thermal conductive substance such as a metal oxide wherein a flexible, deformable silicone layer lies on a silicone resin layer having a sufficient strength to withstand handling. JP-A 7-266356 discloses a thermal conductive composite sheet comprising a silicone rubber layer containing a thermal conductive filler and having an Asker C hardness of 5 to 50 and a porous reinforcement layer having pores with a diameter of at least 0.3 mm. JP-A 8-238707 discloses a sheet in the form of a flexible three-dimensional reticulated or foam member whose skeleton lattice surface is coated with a thermal conductive silicone rubber. U.S. Pat. No. 5,705,258 discloses a thermal conductive composite silicone sheet with a reinforcing sheet or cloth incorporated therein, having at least one surface which is tacky, an Asker C hardness of 5 to 50, and a thickness of up to 0.4 mm. JP-A 9-296114 discloses-a spacer obtained by curing a composition comprising an addition reaction type liquid silicone rubber and a thermal conductive, electrically insulating ceramic powder, and having an Asker C hardness of up to 25 and a thermal resistance of up to 3.0.degree. C./W.
These low hardness, thermal conductive sheets, however, suffer from the problem that when an attempt is made to increase the heat transfer of a corresponding composition by loading it with a larger amount of thermal conductive filler, the composition loses fluidity and becomes difficult to mold.